The subject of the present invention is a reboiler/condenser heat exchanger of the bath type and a heat exchange process in a heat exchanger of the bath type.
More specifically, the invention relates to a reboiler/condenser heat exchanger of the bath type for heat exchange between a first fluid to be vaporized and a second fluid to be condensed, and to the use of this type of heat exchanger. The term xe2x80x9cvaporizationxe2x80x9d is understood to mean partial or complete vaporization and the term xe2x80x9ccondensationxe2x80x9d is understood to mean partial or complete condensation.
This arrangement is used especially, but not exclusively, in air distillation plants of the double-column type in which, for example, liquid oxygen at the bottom of the low-pressure column is vaporized in a bath reboiler by heat exchange with gaseous nitrogen taken from the top of the medium-pressure column.
The operation of bath heat exchangers, because of their intrinsic characteristics, imposes limitations as regards the height for exchange between the first and second fluids or as regards the temperature difference between the primary fluid and the secondary fluid.
This problem will be more clearly understood with reference to the appended FIGS. 1 and 2 which show, on the one hand, an example of a functional diagram showing the operation of a bath heat exchanger and, on the other hand, an example of a functional diagram showing the heat exchange between the primary fluid and the secondary fluid.
FIG. 1 shows, in a simplified manner, the external vessel 10 of the bath heat exchanger, inside which vessel a number of passages 12 for the xe2x80x9cwarmxe2x80x9d second fluid F2 are contained, the said second fluid entering the vessel in the upper part of these passages at 14 and leaving it in the lower part at 16. With regard to the xe2x80x9ccoldxe2x80x9d first fluid F1 to be vaporized, this is contained in the external vessel 10 and flows by thermosiphon effect from the lower end 12a of the passages for the second fluid F2 to its upper end 12b, the height of this heat exchange region being equal to h.
As the diagram in FIG. 2 shows more clearly, the first fluid F1 at the inlet of the exchange region is at a temperature T1-1 and at a pressure P1-1. This Temperature T1-1 and this pressure P1-1 correspond to a subcooling state, that is to say correspond to a temperature below the bubble temperature Tb1 of the fluid F1 at the pressure P1-1 because of the hydrostatic pressure due to the head of liquid fluid F1. This will be shown in the above diagram in which Tb denotes the temperature (the bubble temperature) at which the first gas bubble appears in the fluid F1 during the heat exchange (at an intermediate pressure between P1-1 and P1-1). It will be understood that the energy used to bring the primary fluid to the bubble temperature Tb is xe2x80x9clostxe2x80x9d energy, in order to vaporize the first fluid. Also shown in this FIG. 2 is the second fluid F2 with its entry temperature T2-1 at which it enters the exchange region 12 and its exit temperature T2-2. It can be seen that the subcooling phenomenon results in a xe2x80x9cpinching effectxe2x80x9d in the heat exchange between the two fluids.
Furthermore, the thermosiphon effect, which allows the first fluid F1 to flow, is made possible by the formation of bubbles in the first fluid. If the head in the heat exchanger corresponding to the xe2x80x9cdesubcoolingxe2x80x9d phase is too great, the thermosiphon effect will be insufficient.
It will be understood that the greater the height h of the heat exchange region the greater the hydrostatic pressure on the first fluid at the inlet of the exchange region and therefore the greater the subcooling region must be too. To sustain the thermosiphon effect which ensures flow of the first fluid, the xe2x80x9cpinchingxe2x80x9d phenomenon must therefore be limited. In heat exchange plants of the bath type, this height is therefore limited to 2.5 meters.
Another drawback present in this type of bath heat exchanger is that the xe2x80x9cpinching phenomenonxe2x80x9d described above requires there to be a temperature difference between the entry temperature T1-1 of the cold fluid F1 to be vaporized and the temperature T2-2 of the warm fluid F2 of more than about 1.2xc2x0 C. in order to allow the heat exchanger to operate by thermosiphon effect because of the xe2x80x9cpinching effectxe2x80x9d. However, it will be understood that increasing this temperature difference increases the thermodynamic irreversibilities and, consequently, reduces the energy efficiency of the entire plant. For example, in the case of the distillation of the gases contained in the air using a double column, the pressure in the column called the medium-pressure column and, consequently, the pressure in the feed air compressor, must be increased, thereby increasing the energy consumption of the entire plant.
There is therefore a real need for reboiling/condenser heat exchangers of the bath type or for heat exchange processes in a plant of the bath type which make it possible either to increase the vertical heat exchange height, in order to reduce the floor space of the plant, or to reduce the temperature difference between the first fluid and the second fluid, or else to allow a combination of these two characteristics of the reboiler/condenser heat exchanger.
According to the invention, to achieve this objective the reboiler/condenser heat exchanger of the bath type, for heat exchange between a first fluid (F1) to be vaporized and a second fluid (F2) to be condensed, the said heat exchanger, having a minimum exit pressure Pm,ex of the said first fluid in order to allow the plant in which the said heat exchanger is mounted to operate, comprises:
means for defining a number of passages for heat exchange between the two fluids in order to make the said second fluid flow, the said second fluid having a temperature T2-2 at the outlet of the said passages;
vessel-forming means containing the passage-forming means for making the said first fluid flow by thermosiphon effect between the said passages from the bottom upwards over a height h, the said first fluid having an entry temperature T1-1 where T1-1 less than T2-2 and the said vaporized first fluid having an exit pressure P1-2;
means for giving the entry pressure P1-1 of the said first fluid a value such that the pressure P1-2 is greater than the said minimum pressure Pm,ex and means for ensuring that at least one of the two following conditions is fulfilled:
the height h of the heat exchange passages is at least equal to 2.5 m; and
the temperature T2-2 of the said second fluid is less than T1-1+1.2xc2x0 C.
It has in fact been demonstrated that if the exit pressure of the first fluid is increased, the pinching effect is modified, thereby allowing either the heat exchange height h to be increased or the temperature difference between the two fluids to be decreased.
According to another aspect of the invention, the process for vaporizing a first fluid (F1) using a reboiler/condenser bath heat exchanger comprises the following steps:
a second fluid (F2) is made to flow through vertical exchange passages, the said second fluid having an exit temperature T2-2;
the said first fluid is made to flow from the bottom up over a height h by thermosiphon effect between the said heat exchange passages, the said first fluid having an entry temperature T1-1 (where T1-1 less than T2-2) and the vaporized fraction of the said first fluid having an exit pressure P1-2;
the said pressure P1-2 is given a value greater than the minimum exit pressure of the vaporized fraction of the first fluid needed to allow the plant in which the said heat exchanger is mounted to operate; and
the height h of the heat exchange passages and the temperature T2-2 of the said second fluid are chosen in such a way that at least one of the two following conditions is fulfilled:
the height h of the said heat exchange passages is at least equal to 2.5 m; and
the temperature T2-2 of the said second fluid is less than T1-1+1.2xc2x0C.
It will be understood that this process makes it possible to improve the characteristics of the bath heat exchanger as was already explained in connection with the above definition of the bath heat exchanger according to the invention.
According to a preferred embodiment, the exit pressure of the first fluid P1-2 is about 4 bar absolute, or higher.
According to another characteristic, the height of the passages for heat exchange between the two fluids is preferably at least equal to 3 m.
Preferably, the passages for heat exchange between the two fluids are bounded by parallel plates these possibly being of the type with brazed fins.
According to a variant embodiment, the passages may consist of tubes.
According to a first embodiment, the vessel-forming means comprise a single vessel which contains the said heat exchange passages and through which the first fluid flows by thermosiphon effect.
According to a second embodiment, the vessel-forming means comprise a first vessel defining a lower volume for the entry of the first fluid and an upper volume for the exit of the first fluid and a second vessel connected to the upper and lower volumes respectively, this second vessel possibly being reduced to a pipe.